As a recovery sequence at the time of power recovery in a heat source system having a plurality of heat source machines, there is known a method disclosed in PTL 1 for example. PTL 1 discloses an apparatus for controlling the number of machines in operation that is adapted to control the number of heat source machines. When a power failure occurs, the apparatus determines whether the power failure is a momentary power failure or not. If the power failure is determined to be a momentary power failure, the number of the heat source machines to be operated at the time of power recovery is controlled based on either a load condition or an operating state of the heat source machines immediately before the momentary power failure.